1. Field of the Invention
The present invention relates to a digital nulling and cancellation system, preferably for Global Positioning Satellite System (GPS) receivers, Global Navigation Satellite System (GLONASS) receivers, and spread spectrum radio systems which suppresses inband interference and/or denial jamming signals in the GPS and/or GLONASS L1 and L2 frequency bands using polarization techniques. More specifically, the present invention relates to the reception of orthogonally polarized electric field vectors and to the methods for converting the analog received input signals to multi-bit digital input signals, and to the methods of attenuating interference and/or jamming signals using digital adaptive polarization techniques for mismatching of the antenna feed signal received by the receiver. The present invention suppresses interference and/or jamming by significantly reducing the interference-to-noise and/or jammer-to-signal (J/S) ratio seen by the receiver.
2. Description of Related Art
The Global Position Satellite System (GPS) also called NAVSTAR! is a satellite navigation aiding system which transmits digitally coded data used to determine 2- and 3-dimensional position fixes at a receiving antenna. Its purpose is to provide users with high accuracy position, velocity and universal time throughout the world at low cost. For this reason, control of GPS operability in an interference environment is valuable for both military and civilian applications.
The key to achieving precise navigational performance is the processing of a very weak GPS spread spectrum signal which carries coarse acquisition (C/A) and precision (P(Y)) digitally coded and encrypted data, typically -120 dBm to -136 dBm (isotropic). The GPS signal spectrum uses two L-band frequencies, L1 at 1575.42 MHz and L2 at 1227.60 MHz, with bandwidths of either 2.05 MHz for C/A code or 20.46 MHz for P(Y) code, and employs right hand circular polarization (RHCP) for both L1 and L2 to simplify user dependence on receive antenna orientation. The C/A and P(Y) codes are on L1, the P(Y) code is on L2. Theoretical processing gains for the C/A and P(Y) codes are 43 dB and 53 dB, respectively. The critical GPS receiver reception states are: C/A code acquisition; P code direct acquisition; P code track; P code carrier aided track; and P code direct re-acquisition.
The GPS digital data can be detected and processed even if RF carrier reception is prevented by interference, but high accuracy is attained when the signal carrier is available. This is generally possible because the GPS concept has a high inherent antijam (AJ) capability, however the low receive signal level makes GPS vulnerable to low power interference and/or intentional jamming. It is relatively easy for a local inband source to overwhelm the GPS signal, preventing successful processing of the digital data. As a result the GPS system has several identified susceptibilities and vulnerabilities to interference. From both military and civilian perspectives, it is important to establish an adequate anti-jam capability for GPS systems and ensure availability of this asset in all environments. This was recognized by the military and resulted in the development of several spatial nulling antenna and digital filtering concepts.
Functionally, GLONASS is similar to GPS. Unlike GPS, where each satellite transmits a unique PRN (pseudorandum noise) code pair (C/A and P(Y)) on the same frequency in a CDMA (code division multiple access) format, each GLONASS transmits the PRN code pair at a different frequency. The process is represented as frequency division multiple access (FDMA). Therefore a GLONASS receiver tunes to a particular satellite and demonstrates some degree of inherent interference rejection using its frequency based options. A narrowband interference source that may disrupt one FDMA signal would disrupt all CDMA signals simultaneously. GLONASS also eliminates the need to consider the interference effect between multiple signal codes (cross-correlation).
GLONASS transmits signals centered on two discrete L-band carrier frequencies, L1 and L2. Each carrier frequency is modulated by a modulo-2 summation of either a 511 KHz or 5.11 MHz ranging code sequence and a 50 bps data signal. L1 can vary between 1598.063 MHz and 1608.75 MHz using 20 channels having a 0.5625 MHz spacing. L2 can vary between 1242.938 MHz and 1251.25 MHz using 20 channels having a 0.4375 MHz spacing. The frequency plan is to have satellites on opposite sides of the Earth (antipodal) share broadcast frequencies which has little effect on terrestrial users. GLONASS and GPS both use C/A and P(Y) pseudo random codes to modulate the L1 carrier, and P(Y) only to modulate the L2 carrier. The 511-bit C/A-code is clocked at 0.511 Mchips/sec. The P-code contains 33,554,432 chips clocked at a 5.11 Mchips/sec rate.
GPS and GLONASS receivers exhibit different levels of vulnerability to interference and jamming emitter waveform types, including: broadband Gaussian noise, continuous wave (CW), swept CW, pulsed CW, amplitude modulated (AM) CW, phase shift keying (PSK) pseudo noise, narrowband and wideband frequency modulated signals, etc. Vulnerability is highly scenario and receiver mode dependent. Broadband Gaussian noise is the most critical interference type in the above group because of the difficulty in filtering broadband noise without concurrent GPS or GLONASS quieting, and the intrinsic high cost and performance impact associated with spatial filtering, i.e. null steering, solutions on a moving platform.
A system has been developed for suppressing interference and/or denial jamming signals in the GPS L1 and L2 frequency bands, described in copending U.S. patent application Ser. No. 08/608,493 filed Feb. 28, 1996 now U.S. Pat. No. 5,712,641, entitled Interference Cancellation System for Global Positioning Satellite Receivers, inventors being Casabona, Rosen, and Silverman and assigned to the same assignee as the present application (hereinafter the "Casabona I application") and described in copending U.S. patent application Ser. No. 08/713,891 filed Sep. 17, 1996, now U.S. Pat. No. 5,822,429 entitled System for Preventing Global Positioning Satellite Signal Reception to Unauthorized Personnel, inventors being Casabona and Rosen and also assigned to the same assignee as the present application (hereinafter the "Casabona II application"). Such system employs polarization nulling utilizing electric field vector cancellation to effect inband interference suppression for GPS and GLONASS systems. Polarization cancellation has also been known to eliminate interference signals in data links and for communications channels, and for robust radar electronic countermeasures and electronic counter-counter measures. See, U.S. Pat. Nos. 3,883,872; 4,283,795; 4,937,582; 5,298,908; and 5,311,192. The general implementation of polarization in GPS systems, as described in the Casabona I and II applications, uses a dual polarization antenna, a hardware polarimeter network and a control loop to cross-polarize the antenna network to interference of the composite signals. The general implementation of polarization nulling in communications utilizes a tracking channel to track the interference signal in phase and amplitude and reintroduce this signal in a canceling circuit to cancel interference components of the composite received signal. RF polarimeters have also been utilized in instrumentation radars to realize antenna matching, optimize performance, and for target measurement. Reciprocal RF polarimeter devices are utilized for radar jamming to realize cross-polarization countermeasures. Polarization nulling as used in the Casabona I and II applications for GPS interference suppression applications utilize a hardware implementation of the polarimeter structure composed of separate phase shifters and hybrid junction devices to suppress wideband and narrowband interference.
Digital adaptive transversal filter nulling for spread spectrum receivers as an approach to cancel narrowband interferences is known in the prior art. See, U.S. Pat. No. 5,268,927. The generalized implementation digitizes analog input signals, which comprise multiple spread-spectrum signals, thermal noise and additive multiple interferers, and applies a digital finite impulse response (FIR) filter response to the multi-bit digital representation of the input signals, and uses a set of variable digital weight coefficients to generate digital output signals which contain a reduced amount of narrowband interference. A significant problem is that adaptive transversal filtering is not effective in processing wideband interference or jamming without disruption of the underlying GPS signals. Adaptive transversal filtering is very effective against continuous-wave (CW) interference and narrowband interferences, such as pulsed CW and swept CW. Polarization nulling, in comparison, is effective against all forms of interference, especially wideband noise interference.
It is thus desirable to provide a digital signal processing interference canceling system for GPS systems that can deal with complex narrowband and wideband interference environments composed of diverse interference and/or jamming waveform types, L1 and/or L2 band interferences, multiple interference sources, and different interference polarizations. It is further desired that the interference canceling system provide high levels of cancellation for either or both of the GPS operating frequencies and adapt to variation in orientation of the receiver antenna(s) and/or the interference source. It is desirable that the polarization interference canceler process digitally encode representations of the received signals and implement the polarization signal cancellation phenomena on these signals, preserving the information content of the GPS signals.